1. Field of the Invention
The present invention relates to a melamine reactor according to claim 1 and a process for obtaining melamine according to claim 13.
2. Description of the Prior Art
Melamine is usually obtained from urea in the presence of ammonia according to6H2NCONH2+nNH3→C3N3(NH2)3+6NH3+3CO2+nNH3 
Thus for every mole of melamine three moles of CO2 and six moles of NH3 together with the introduced NH3 in excess are obtained.
The industrial processes for melamine synthesis are mainly classified into a catalytic process at low pressures below 1 MPa and non-catalytic processes at high-pressures above 5 MPa (see Ullmann's Encyclopedia of Industrial Chemistry, 6th edition, Vol. 21, p. 205 ff).
In the following reference is being made to the high-pressure processes and in particular to the process steps carried out in the high-pressure part of a melamine plant.
At present the high-pressure melamine process starts out from pressurized molten urea at temperatures between 135 and 160° C. and is carried out in a pressure range between 5 and 20 MPa and a temperature range of 370 to 430° C.
The high pressure part comprises basically three sections, in which the following process steps are carried out:                endothermic reaction of urea to melamine,        separation of the offgas from the melamine melt, CO2 removal by introducing ammonia gas in the melt and optionally aging of the melamine melt, and        washing or scrubbing the offgas.        
These three different steps are performed in different sets of equipment separated from each other. The conversion of urea to melamine is carried out in a synthesis reactor with heating bundles. The obtained raw melamine melt is then transferred to a second reactor or stripping reactor in which the dissolved gases, in particular CO2, are removed from the melamine melt by introducing gaseous ammonia in counter current, and where the melamine melt is allowed to reside for a certain time (Aging).
The offgases generated during the melamine synthesis reaction are usually removed in the upper part of the melamine reactor and then transferred to the scrubber. The raw melamine melt is introduced in the second reactor where ammonia gas is introduced in counter current for removing CO2 from the melt which is also transferred to the scrubber. During the stripping in the second reactor the melamine melt is allowed to reside for a certain time (Aging).
In order to reduce the amount of side products, in particular melem and melam, liquid or gaseous ammonia can be introduced to the reaction melt. The ammonia is fed through a suitable inlet, in particular through a distributor, injector or nozzle.
The offgas is removed from the stripper and sent to a scrubber where the offgas is cooled, melamine present in the offgas is separated from the offgas, and in counter current the urea melt is preheated. The urea melt comprises fresh urea or circulating urea which is kept in circulation by a circulation pump. This urea circulation pump serves also for feeding the preheated urea melt to the high pressure melamine reactor. The excess heat in the scrubber is removed by an external heat exchanger. All in all, a conventional high pressure part of a melamine plant consists of approximate 5 different main pieces of equipment, in particular synthesis reactor, stripper, scrubber, heat exchanger and circulation pump, which are connected by a number of pipes.
Thus, this construction requires a number of different bulk materials like pipes including the proper insulation, and intensive heat tracing, valves, steel structures and more. Such a complex system of reactors, pipelines and valves is however difficult to operate and to maintain.
Considering that the applied pressure is above 5 MPa it is obvious that a reduction in bulk material, in particular a reduction of pipes and valves would minimize the risk of failure and would simultaneously reduce the costs for equipment (Capex) and maintenance and plant availability (Opex).
Thus, it would be desirable to simplify the construction of a high-pressure part of a melamine plant.
One possible approach is introducing specific improvements into the equipment design (U.S. Pat. No. 3,700,672) what however increases the overall investment costs.
Another approach is the combination of the different reaction steps e.g. melamine synthesis, stripping, scrubbing in one reactor. Such a reactor is for instance described in EP 612 560 A1. This high-pressure melamine reactor comprises three sections separated from each other by means of diaphragms: a top sector serving as washing or scrubbing sector, a central sector serving as stripping or separation sector, and a bottom sector serving as synthesis reactor. Each sector communicates with the other two sectors through pipelines.
However, this reactor still requires a number of different equipment like diaphragms, pipelines and valves making it vulnerable to corrosion by the processing fluids or melts and causing high costs.